


Waterborne

by Alyndra, deli (deliciousirony)



Category: Supernatural
Genre: Dean more or less holds Castiel captive but not against his will, First Kiss, First Meetings, Human Castiel (Supernatural), M/M, Merperson Dean Winchester, Power Imbalance, Supernatural Reverse Big Bang Challenge 2019, Underwater, a little nonconsensual body modification, and worldbuilding, but really it’s fluff
Language: English
Status: Completed
Published: 2019-12-07
Updated: 2019-12-07
Packaged: 2021-03-03 20:22:30
Rating: General Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 6,019
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21696361
Author URL: https://archiveofourown.org/users/Alyndra/pseuds/Alyndra, https://archiveofourown.org/users/deliciousirony/pseuds/deli
Summary: Castiel's spent his life looking for traces of the mysterious sea people - science experiments from an ancient time humanity is only beginning to rediscover. Just when he's about to drown alone, he's rescued by a real live merman and brought to an underwater sanctuary. But what does the merman want with Cas?
Relationships: Castiel/Dean Winchester
Comments: 5
Kudos: 63
Collections: 2019 Supernatural Reversebang Challenge





	Waterborne

**Author's Note:**

> Thank you [Deli (deliciousirony)](https://archiveofourown.org/users/deliciousirony) for first of all drawing such wonderful, gorgeous art that it inspired me to write Destiel for the very first time ever, and secondly for brainstorming up this world with me, and then for being so patient while I wrote it in tiny little bits and pieces! You can admire and reblog [here!](https://www.pillowfort.social/posts/955485)
> 
> And thank you ever so much [BlindSwandive](https://archiveofourown.org/users/BlindSwandive/pseuds/BlindSwandive) for being a great beta - ❤️❤️❤️, lovely!
> 
> And finally thank you to [Beelikej](http://beelikej.dreamwidth.org/) for running the [spnreversebang](http://community.livejournal.com/spn_reversebang/) with superb flair, here's to many more!

* * *

Castiel took a deep breath, which was somewhat miraculous, considering that there was an ocean on top of him. He lay on a clear, smooth, strangely rubbery floor, with water below it. There were clear walls and a clear ceiling, too, all surrounded by water, and through some of them he could see more barely-visible structures, towering clear columns and dizzying sweeps of architecture.

But his floor only covered part of the room he was in. Right in the middle, there was a wide opening and the ocean rippled bare and dark there, still clear enough to see through.

He could see the big, powerful shape of the merman, as he burst up through the water and flopped himself across the other side of the floor. Human-looking head, arms, torso, check. Massive fishy tail and gill slits all down his sides, also present. Cas couldn’t help staring.

“Am I what you expected?” the merman asked, in perfectly intelligible—old-fashioned, maybe—Standard. It was like listening to a program from hundreds of years ago, or longer. Cas tried to make his own words match, but they just came out sounding stiff.

“I—No, I did not expect—I mean, there have been rumors, legends, but no one has seen anyone like you for—”

“Take it easy,” the merman short-circuited the babbling, looking amused now. “Hello. I'm Dean Winchester.”

“Of course. My name is Castiel,” he hurried to introduce himself in return. The last thing he wanted was to be rude. “How many of you are there?”

Dean’s smile flickered, like he didn't like the question. “I’m one of a kind, Castiel. Why're you so curious about merpeople?”

“It’s my life’s work,” Castiel said simply. “Ever since I heard stories of the Sea People who were created in the last great Age of Exploration, formed out of humans but given the ability to breathe and swim underwater, I’ve tracked down every reference I could find to them, to the scientists who—” he was forgetting to speak slowly. He paused, and became aware that Dean was not sharing his enthusiasm. “I managed to track down the last known lab site’s location, well enough to start looking, at least...”

“And here you are,” Dean finished for him, pure edged sarcasm. “Good job. Do you want a cookie?”

Castiel was afraid he was gaping. “What? No, I don’t want a...a sugary snack. Would you rather I just left, then? If your people don’t want to be bothered? You didn’t have to bring me here.”

“Yeah, I could have let you be eaten by a shark,” Dean rolled his eyes. “Please. If you’re going to capsize your boat in a thunderstorm, at least don’t get your hand cut open in the process.” He reached out and took Cas’s hand, which had at least stopped bleeding, but the cut across his palm hadn’t been out of the water long enough to start to scab over yet. He examined it closely, looking for debris, maybe, and then covered it with his own palm, pressing gently against the wound. “And don’t swim off on your own away from your crew. Don’t even the most landbound humans know blood attracts sharks?”

“I did know,” Cas admitted, leaving his hand in Dean’s even though it wasn’t exactly comfortable, having the cut touched. It might have some sort of cultural significance to the merpeople even if it wasn’t good hygiene. He couldn’t resist tilting their joined hands to admire the long, webbed fingers, though. It looked so strange, covered with normal human skin, the length and thinness of the fingers just outside of what was normal for humans. “I didn’t want to draw the sharks to the other survivors, so I told them I was going to try to swim to the nearest shore for help.”

“I went back to find them after I brought you here,” Dean said casually. “But they were all being picked up by helicopter when I found the boat. If you’d stayed with them, you’d have been fine. Probably on your way home by now.”

“Then I guess I made the right choice,” Cas scoffed, feeling curiously defiant. “I came out here to learn what I could about you and your people. You can’t think I’d rather be going home than sitting here talking to you?”

Dean released his hand. “Yeah, well, that doesn’t mean I have to sit here and talk to you.” He dove into the water, and Cas watched, dismayed, as the shape of the merman swam out of sight, down through the maze of the palace. He couldn’t follow, not without knowing where his next breath would come from.

Well. That had been...interesting. “Are you just going to leave me here to starve?” Cas called after him, way too late. But when he looked at his hand, he had to stare: there was new skin covering the cut, still red and tender but getting less red as he watched.

What the hell?

The merman was back before the sun set and it got truly dark, although it was getting noticeably dimmer. In a clear case, he brought actual sushi rolls, raw fish wrapped in seaweed and something enough like rice that Cas was surprised.

“Hey, rice grows in water, you don’t think any of it washes downriver?” was all Dean would comment, though.

“You’re fucking with me,” Cas said, eyes narrowed.

“Yep,” Dean admitted cheerfully. “You’d be amazed at the places a big city will deliver to.”

Castiel tried to decide if this was any more truthful than the last explanation. Dean's smirk didn't bode well. But then Cas' stomach grumbled loudly. Being immersed in cold water burned a lot of fuel, and he sort of suspected that skin-growing trick Dean had used on him earlier did, too, it was just hard to tell when he was already hungry enough to eat an entire raw fish. “Thank you for coming back,” he said. “It’s not my intention to cause trouble for you.”

“No problem,” Dean shrugged. “What kind of asshole would just leave you here to starve?” He shoved the container across the floor to Cas, and it slid easily on the membranous surface, though its slight weight made the floor curve under it.

Cas frowned as he picked it up. “Why does the floor bend more for something under a pound than it does under my feet?” he asked, turning over the package to try and figure out how it opened.

“Your feet are bare,” Dean said, and Cas looked down. He’d kicked off his shoes, of course, to swim better, and nobody liked wearing just wet socks so he’d ditched those too. “So they transfer electricity through the skin when they touch things. The membrane that makes up this place"—he reached out and rapped his knuckles against a wall, though Cas distinctly remembered seeing that wall sway, before—”it’s malleable without an electric current going through it, but hardens up locally if even a small charge is applied. The amount of electricity in you or me is more than enough.”

“That’s amazing,” Cas said sincerely, food forgotten in his hand. “Was that technology that the ancients had? The ones who first created your people?”

“No,” Dean said. With a flip of his tail, he heaved himself up out of the water and scooted over next to Cas. The floor stayed solidly floor-like under him. “They built labs here of glass and metal and plastics to keep out the sea. But after they left and it was up to us to maintain the structures, or else let them collapse, we found better ways than they used.” He took the package from Cas’ unresisting fingers, and the cover fell apart in his hands; Cas couldn’t at all see how he’d done it. “Here, eat.”

Cas took the bite of sushi he was handed and put it in his mouth automatically. “But then how—what if I was wearing shoes?” he asked plaintively. “And how does the structure stay…?” He swallowed, savoring the delicate taste, although he was hungry enough that any food would taste like manna from heaven. But this was truly...he looked down at Dean, sitting on the floor next to him, surprised. “That’s delicious, actually.”

“Have some more.” Dean shoved another piece into his hand. “If you wore shoes, you’d bounce, like a giant trampoline. Maybe get seasick, even—humans are weird about some things. Not recommended.”

Cas sat down, carefully, a couple feet from Dean, still chewing the second piece. He was still wearing trousers, but with his hands and feet on the floor, it stayed hard enough to support him. He expected that much, frankly, from having sat down earlier. But when he raised his feet and hands so that only his butt was touching the floor, it immediately sloped away from the island of electrical stability that Dean was providing and he slid over onto his side, yelping even though he expected it, intellectually.

Dean just sat there grinning, like watching a kid play, and it _was_ kind of like being on a waterbed for the first time. Cas put his hands down and the floor froze into the new billowed shape it was in, with him at the center of a hollow and a wave around him, except where Dean’s chunk of floor had stayed flat. He clambered out, towards Dean, and once his feet were on a relatively level spot again watched as the hollow settled back into place, like a room-sized pond with waves getting smaller and smaller until it was still again.

He tried to reach outside the circle electrified by his feet, at first slowly and then quickly, but the reaction was close enough to instantaneous that the only way he could feel any give was if he wrapped fabric around his hand first.

“If I leave you here like this, are you going to forget to eat?” Dean asked, sounding amused, and with a start, Castiel was reminded that he was, in fact, blazingly hungry.

“No,” he said with as much dignity as he could. He sat down next to Dean again, because it didn’t seem polite to keep towering over him, although Dean didn’t give any indication of minding. Maybe merfolk thought it was ordinary to talk to someone floating over or underneath you instead of staying on the same ground level all the time. “May I have the rest?”

Dean handed the sushi roll over and watched Castiel interestedly. “Do you like fish? Normally, on land, I mean. I know some humans don’t like the taste.”

“Yes, I used to go fishing with my father,” Cas answered absently. He glanced over at Dean. “Before he disappeared. I suppose...I thought if I could learn the secrets of the ocean, it would make up for never knowing what happened to him.”

“That sucks,” Dean said, with sympathy. “My dad’s a pain in the ass but at least he’s still around to keep my nose to the grindstone.”

“I’ve never heard that phrase before,” Cas said, willing to be distracted. “Is a grindstone like a whetstone?”

“I don’t know,” Dean said, surprised. “I don’t think it means anything, it’s just something people say.”

“You speak very normally, for someone whose people have been out of contact with humans for hundreds of years,” Cas said, hoping to get Dean to talk more about...himself? His people? Either, if he was being honest with himself.

“Well, we try to keep up,” Dean had that flippant tone in his voice again. “You humans broadcast an awful lot of radio signals. We figured out how to pick up television a long time ago.”

“I’m sorry,” Cas automatically winced. “For all the commercials. And politics. And probably everything, really.”

Dean waved a hand. “You don’t have to be sorry for free entertainment, are you kidding me?”

“Well,” Cas allowed himself a cautious smile, “okay then. What’s your favorite?”

“Dr. Sexy,” Dean said promptly. “And before you ask, I watch it for the plot.”

“...I would not have questioned your reasons,” Castiel said. Was Dean _blushing_ now? Fascinating.

“I should probably get going, anyway,” Dean shrugged. “I’ll be back in the morning with more food.” He arched his tail up in a sudden smooth motion and sent it slapping down against the floor in a motion that catapulted him easily within arm’s reach of the hole open to the ocean, and from there it was a quick grab and pull and he was in the water.

“Wait, I—“ Cas reached out belatedly.

“Yeah?” Dean’s head popped up out of the water.

“Why…” Cas tried to formulate what he wanted to ask. “Why are you okay with me staying here?”

Dean grinned. “Maybe I’m just as curious as you are.”

“And your father? The others—your people? What would they think of me?” Cas asked. He might be pushing too much, but he had so many questions, and he didn’t think he could sleep if he didn’t at least try to get some answered.

Dean sighed, brow furrowing as he clearly thought about what to tell Cas. “My father, all our people, have always known that we are Legacies,” he said slowly. “A remnant of a forgotten time, from technology humanity could not work today if it wanted to. It’s our job to protect the world.”

“From what?”

Dean shrugged. “From knowing too much, too fast. From forgetting entirely. From totally ruining the biosphere, or at least the biggest part of it.” He gestured at the ocean around them, and the swooping architecture of the merpeople's habitat, swaying in the currents. “Maybe even from ourselves. We’re still not that different, when it comes down to it.”

Castiel looked out at the view thoughtfully. “There can’t be all that many of you, compared to humans,” he said. “If there were a billion merpeople in the oceans, you wouldn’t be able to hide, not indefinitely. Something the size of this structure would take many people to maintain, but I haven’t seen a single one the whole time I’ve been here, and I’ve been looking.”

Dean waited to hear what Cas thought, face shuttered, wary.

“So I think maybe either there’s very, very few of you, or else somebody told everyone to stay away from me,” he finished, suddenly sure as he saw Dean’s reaction. “They all know I’m here, don’t they?”

“Yeah, okay,” Dean admitted. "They do."

“So when do I get to meet them?” Cas pressed, unable to help it.

Dean looked at him strangely. “You really don’t care if you get to go home, do you? Most people, if I brought them down here, they’d be asking how they could get back to the surface, back to land. You haven’t brought it up once.”

Cas waved that away impatiently. “I can’t believe that anyone would get this close, actually meet a real live merperson, and want to turn back! There’s still so much I want to learn about your people.” His breath caught. _About you,_ he’d almost said, and, well, _that_ was unexpected.

“Even if learning too much means you can never go back home?” Dean asked bluntly, completely serious again. “Because that’s at stake here, wonderkid. Think about it.” And with a casual salute of his arm, he disappeared into the water, leaving Cas alone in the deepening gloom, membranous walls rising high all around him.

* * *

* * *

It was true. Cas had spent the rest of the night thinking about it, in between snatches of restless sleep. It was chilly, but the membranous material seemed to insulate his body heat fairly well from the surrounding ocean, and even wicked moisture from his clothing, given long enough. It was amazing; the place must never have to worry about small leaks, if it automatically returned water to the outside. The way Dean had healed his hand, this very structure he'd been left in - no human technology could even come close to any of it.

For all that Dean didn’t look like someone from a high-tech society, that didn’t mean he wasn’t. And that meant the merfolk had to be used to guarding their secrets from humans, which meant...what? Would they kill Cas to keep him from revealing them? Or simply leave him in this room the rest of his natural life?

But Dean had implied there was still a choice to be made, right? Because Cas hardly knew anything of them yet. They could let him go and all he would have to tell other humans would be a crazy story and a general location, and if they were capable of rebuilding their palace somewhere else—which they probably were—then they would risk almost nothing by letting Cas go. Unless they did let Cas find out more about them, first.

All of which just made Cas want to know _more,_ passionately and stubbornly. What would he be leaving behind, if he never returned to his life? He had no lifemate, no children. No parents, not for many years. Siblings, yes, but they were all grown, with their own lives and careers. They would miss him, and he them, but…

His career was the driving force of his life, and his career _was_ researching the ancient legends of merpeople created by the ancestors. How could he go back to any of it, knowing what he knew now? Knowing he’d had the chance to learn more, to surpass all his field’s wildest dreams, and that he’d backed away? He _couldn’t._ He couldn’t refuse whatever opportunities there were here, if he could only get the merpeople to trust him.

And Dean—when he thought about Dean’s grinning face, the warmth of the short companionship they’d already shared, there was something unique about the experience...by comparison, all his colleagues seemed dull and far away. If those colleagues were the only ones he got to talk to the rest of his life, he would be bored out of his mind, he felt. Dean was different from everyone else he’d ever met, somehow. He wanted to spend more time with him. He couldn't imagine ever getting his fill of this strange person, and he wanted to find out _why_...

No, there was no way Castiel could agree to turn back now, and that was all there was to it.

Dean joined him as light turned the sea shades of lavender and rose. Dawn. He brought a food packet of seaweed and coconut with tiny shrimp this time, nothing like anything Cas had ever eaten before, but still delicious.

"The best time to look for coconuts is on a flat calm day after a storm, leeward of the right kind of island," Dean volunteered, when Cas asked. "They can keep a long time in the water, of course. That's what they evolved for, to spread coconut trees from island to island by floating around the ocean."

Cas hadn't ever particularly thought of them as an ocean food before, but it made sense. "Seems like there are a lot of things that wind up in the ocean one way or another," he said.

Dean picked up the cue effortlessly. "Like you, right, Castiel? Have you thought about whether you want this to be a short sea adventure, in between putting down roots on land? Or are you gonna let yourself fall into the hands of the merpeople for good?" Dean's eyes weren't laughing at the moment. "Because we take good care of our treasures, Castiel. But once we've got a coconut in our nets, we don't just let it drift off out of sight again."

"Yes, I get your point," Castiel snorted. "And I have thought about it. I've thought of little else since you left last night. But I can't imagine anywhere I'd rather grow that would be better than learning more about your people. I want to spend more time with you." Castiel half-wished 'you' wasn't such an ambiguous word, and then he wondered if he'd really meant Dean's people, or Dean himself. "If that means all the time I have left before I die of old age, so be it."

"So be it," Dean echoed. It had an air of finality. He spat on his hand and stuck it out. Castiel barely recognized the old-fashioned gesture in time to respond in kind, but Dean didn't seem bothered by his unfamiliarity. He gripped Cas's hand tightly and held it for a long breath. "There. Changes have been set in motion, now; hope you don't have regrets later. Come on, let me show you around the place."

Cas eyed the open water which was the only way in or out of the room a little dubiously, but if Dean wanted to take him there he’d go. “Do other parts of this place have air in them?”

Dean grinned. “Yeah, the membranes passively filter water out and oxygen in, so it builds up over time. We actually have to distribute it so it doesn’t accumulate enough to threaten the structure.” He slid over to a wall and pressed his hand against it; after a moment, a fissure appeared, much to Cas’ surprise, and opened to reveal a tunnel. An air-filled tunnel.

“I would have noticed that, yesterday!” Cas protested. “The walls are _clear,_ and air looks different than water!”

“Well, it wasn’t there yesterday, was it, then?” Dean asked. “I just asked her to bring it over for us.”

“Her?” Castiel asked.

“This place, this habitat or repository or whatever you want to call it,” Dean said. “I’m the one who spends the most time here, making sure the structure is healthy, going over the old records. I guess you could say she’s sort of my baby.”

“In that case,” Cas said, “I am honored to meet her.” He gave a short bow.

“Most people think I’m weird to talk about her like a person,” Dean admitted, looking embarrassed. “She’s not, really. She responds to directions, but she’s not sentient.”

“I like hearing you talk about her,” Cas smiled. “And everything here is new to me, so please keep talking.”

“Yeah, alright,” Dean muttered. “This way. After you.”

Cas needed no further prompting; he was nearly vibrating with curiosity to see where the tunnel led. It was just tall enough for him to walk straight in, rounded but narrower than it was tall, and slightly flattened on the bottom. He walked in and Dean followed him by somehow propelling himself over the floor in a smooth slide, not at all like awkward seal lunges or a person trying to move without use of their legs.

“You’re making the floor help you!” Cas exclaimed eventually, half outraged and half impressed. It was rolling forward in a long smooth tilted wave right behind Dean’s weight, so that really Dean was just sliding constantly downslope at a pace comparable to Castiel’s walk.

“I could teach you the trick of it,” Dean grinned, “but you’ve been practicing walking a lot longer, and I do want to get places today.”

They were in a long arching walkway that curled up the inside wall of one of the tower-like structures. It gradually ramped them higher until they reached near the top of the domed tower, and abruptly their heads were coming out into an open-air room, much larger than where Cas had stayed the night. There were pools of water strategically placed where humans might have had couches or chairs, but there was no mistaking the purpose of the tables and shelves: this was some sort of workroom or library. Or, perhaps, a museum?

There were a few real books on the shelves—incredible, if they hadn't been ruined by water—and some ancient computers, which try as he might Cas couldn’t imagine still functioned. Perhaps the books were filled with nothing but waterlogged pulp, too. But most of the space was taken up by bric-a-brac of a wild assortment. Pens, coins, sculptures—there were a great many busts in a unique style, all of different faces, but it wasn’t until he tried to pick one up that Cas realized it was made of hard coral, not stone or clay.

“We shape the coral, as it grows, into the face,” Dean explained. “It’s an exacting art form. These are all a type of coral that has been bred to grow into the right general shape of a human head, and to accept instruction easily. There are other varieties for full-body statues—mermaid and human—and some for pairings, even.” He winked so lewdly that even Cas got the picture and hastily nodded, blushing. Of course merpeople would have lewd statuary. Most cultures did, at one time or another.

“The books and computers look like they’re human-made,” Cas remarked, to change the subject.

“That’s because they are,” Dean said. “Those things were left here when humans abandoned their undersea labs.”

“They’re in remarkable condition,” Cas said. “I’ve hardly ever seen materials that old in such good shape.” His fingers itched to handle them, read them, discover secrets buried in those pages and circuitry, but no, he wouldn't just grab—not without being invited.

“You can touch.” Dean sounded amused again. “All the data here has been copied elsewhere, many times over. We just keep this stuff here for historical value, really.”

“Do you have waterproof paper to copy them to, then?” Cas asked absently, already reaching out for a copy of a genetics text that he’d only ever been able to find in pieces, pages torn out, damaged.

"Better," Dean said. "We just encode it into DNA. Easy to store and copy that way."

"But...how do you read it?" Cas frowned, looking from the textbook—easily the weight of a newborn baby—to Dean, lounging casually next to him, naked as the day he was born. Dean hadn't carried or worn so much as a wristwatch any of the times Cas had seen him, although obviously, in order to have created and maintained this place, his people had to be fairly technologically advanced…

"We coded ourselves to be able to consciously process any DNA we can taste," Dean said, all nonchalant, and blew Cas' mind. "I read your DNA when I first brought you here and kissed your hand. It's a lot of data sometimes, but you get better at seeing the patterns with practice."

Cas sat down on the floor abruptly. He'd never heard of anything like—never even imagined. What would it mean to have that much information at—at the tip of your tongue, literally? "_I_ don't even know what's in my DNA," he said, stunned. "I mean, sure, I've been scanned for a few genetic diseases, but all the stuff in between?"

"You'll be pleased to know you're in quite good health, genetically speaking," Dean said, eyes crinkling. "No ultra-rare ticking time bombs. Although even if you had some, you wouldn't have to worry; those kinds of things are easy to cure once you know what you're doing."

"Modifications," Cas said slowly. "Everything I've seen you do—healing my hand, shaping this place, the food you brought me, even the _wrappers_—those are all from your ability to modify the things around you at the genetic level?"

“Yeah,” Dean nodded. “You're picking up quick. Gonna teach it all to you, over time. You know, since you decided you’re staying.”

“You mean... I could learn…” Cas’ breath caught. It was something beyond his wildest dreams, being offered to him on a silver platter; there were too many questions he had to ask all at once. “How do I start?”

“Taste the floor,” Dean said, and Cas blinked, taken aback. “Or a wall, they’re the same makeup, it doesn’t matter.”

"But…" Cas blinked at him. "I'm just plain human, I wouldn't be able to taste anything like you can…"

"When you decided to stay, you gave up being just an ordinary human," Dean said. "I told you I set changes in motion. You've got what you need to be like me now, to do what I can do. But that's just the easy part. Learning to use them takes training."

"You mean...when you spat on my hand…" Cas felt spun about. Should he have realized earlier?

"Yeah, exactly," Dean confirmed. "I generated a virus-enclosed packet of DNA that would modify your genetic makeup, tuned to you based on the read I got off you when we first met. You're going to be able to taste genomes, eventually create your own modifications, and in the meantime, I figured you'd be more comfortable if you weren't an obligate air-breather, so I gave you the basic gills-and-fins package." Dean's words were casual, but he was keeping a wary eye on Cas as he spoke, as though waiting to see if Cas was going to freak out and explode.

Freaking out had a lot to recommend it, Cas supposed. "I'm going to turn into a merman like you?"

"I mean, I'm one-of-a-kind, remember?" Dean half-grinned. "Your legs wouldn't join together and turn into an extended spine, not without some seriously invasive mods. But lengthening and webbing the feet, and your hands, that's not so difficult, and we can always undo it later if you don't like it."

"Okay," Cas said carefully. Dean wasn't meeting his eyes, like he knew Cas had every right to feel upset by this, no matter what cultural differences lay between them..."Is it customary among your people to modify another person without asking?"

Dean looked even more distinctly uncomfortable. "Kids too young to do their own mods, yeah. And even that's not so much because mods are difficult, as...well, who wants a five-year-old with the power to release any virus they wanted into the ocean? Kids have temper tantrums. Working with living things, not everything can be fixed, you know."

"I think I see," Cas said. "So you will treat me as a child among your kind, then."

"No," Dean said, making a frustrated chopping motion with his hand. "I knew I was going to fuck up explaining this. Dammit. I tried to get my brother Sam to come help, but…"

"In some ways I will be like a child. I understand. Even your five-year-olds will know more of this—technology—or whatever you call it—of yours, for some time," Cas said, staring around at the artifacts of ancient science surrounding them. Humanity had lost so much, in the interval between then and now: modern society could not have recreated most of these things without a great deal of effort. Sometime in the next hundred years, perhaps, they would reach and surpass the level the ancients had reached, but they were still striving to rediscover everything they had lost. And now to find out that the merfolk had never lost any of it, that they had kept _advancing_, at least, in genetics science…

"You staying here with us is non-negotiable at first, because you can't chicken out and run back to humanity, not until you've mastered what we have to teach you," Dean interrupted flatly. "The stuff we can do, it's too dangerous a power for anyone to have if they don't know what they're doing with it. I can't tell you how many times we nearly set off plagues that destabilized the world ecosystem, and I don't need to tell you that it's barely recovered from humanity's _first_ attempt at global civilization. We do what we can to keep it all healthy, now, of course, but I'm not joking around: if several billion humans were all trying to figure out how to use this stuff at once, they'd almost certainly wipe out all multicellular life on the planet before you could think twice. That's how powerful it is."

Cas gulped. He hadn't thought quite that far. "So then why...why save me at all? Why bring me here or give me this?"

"Because humanity will get there on its own eventually, if it stays on track without going into another downward spiral," Dean sighed, and tossed one of the coral busts onto the floor, just far enough away from them both that it sank into a divot with the weight, and then bounced back up as the water rushed to make a peak where the impact had been. It rolled off and away as Dean used a quick thrash of his tail to get on top of the little peak—knee height on Cas—which brought him up to where he could look Cas in the eye, with his tail curled around the hummock for stability. Of course it had frozen as soon as Dean touched it. "And it'll go a hell of a lot better if there's someone marking out the trail ahead of them. We were never meant to stay secretive forever, you know? Just until the world was ready to be responsible with this knowledge."

"I see." Cas took a deep breath. It was unnerving to have Dean suddenly the same height as he was—if Dean had had human legs, he would even be taller than Cas, he realized suddenly, and Cas wasn't considered short. "You want me to learn enough that I can potentially be an ambassador between worlds, between the humans and the merfolk—but not until I spend enough time with you for you to trust me without question."

"It may never happen," Dean said. "You could prepare your whole life, we could do everything right, and if there's too much sociopolitical instability on land, my people could decide not to reach out to humanity within either of our lifetimes. We live a long time, but even we age and die eventually."

"That's why you had to know if I was okay with staying with you indefinitely," Cas nodded, putting it together. "I understand. It's okay, my answer is still yes."

"I get it that you might not want to...wait, what?" Dean started to slide a bit and Cas reached out and caught his arm.

"You're not going to drive me off, no matter what you want to do to me," Cas said, feeling an odd kind of...certainty? happiness?...bubbling up inside him. "I like you, Dean Winchester, and I don't want to go anywhere."

"You…" And to Castiel's immense amusement, Dean actually blushed. "You can't know everything I might want to do to you," he muttered.

"That's true," Cas agreed, faux-thoughtfully. "But I can think of a few things I wouldn't mind at all." Quickly, before he could overthink it, he leaned in and pressed his lips to Dean's.

Dean froze in startlement for an instant, and then—wonder of wonders—he relaxed, and let it become a real kiss. He was smiling when he pulled away. "You're insane, you know that, Castiel?"

"Yes, well, don't start complaining about it now," Cas said. Dean's taste was still rolling over his tongue, traces of complexity he couldn't yet understand starting to draw patterns in his mind. DNA didn't taste like anything he could have imagined, but he could already tell it would be easy to spend hours contemplating it. Later, though. "Anyway, when do I get to meet your family?"

Dean threw back his head and laughed. "Let me know when you feel like going swimming. Sam's going to love you—he does a great impression of a pilot whale, by the way, he's big enough to pull it off and the whales all like the noises he makes."

"Pull what off? Does he actually disguise himself, in some sort of skinsuit?" Cas asked. "Can you do that? What about selkies, do you disguise yourselves as seals sometimes, too?"

Dean slid off the raised patch of floor, laughing. "Yeah, I think you're going to fit in just fine," he told Cas. "C'mon, let me show you how to make an airlock."

Cas followed him over to the wall. What the hell. He leaned over enough to lick the strange rubbery surface. "I look forward to learning," he said, and let the secrets of Dean's 'baby' start to unfold inside his mind.


End file.
